Spoofed versions of the popular file transfer program FileZilla that steal data are circulating on third-party websites, the organization behind the software said Tuesday.

FileZilla is an open source application, and hackers have taken its source code and modified it in order to try to steal data for more than a decade. But this campaign, run on third-party websites, is one of the largest FileZilla has seen to date, it said.

“We do not condone these actions and are taking measures to get the known offenders removed,” FileZilla said.

The organization said it is difficult to prevent tainted versions of its software “since the FileZilla Project promotes beneficial redistribution and modifications of FileZilla in the spirit of free open source software and the GNU General Public License.”

The security vendor Avast found that the modified versions are nearly identical to the legitimate application. The icons, buttons and images are the same, and the malware version of the “.exe” file is just slightly smaller than the real one, Avast wrote on its blog.

Inside the tampered FileZilla versions, Avast found code that steals login credentials for servers users are accessing. The username, password, FTP server and port are encoded using a custom base64 algorithm and sent to the attacker’s server, according to Avast.

“The whole operation is very quick and quiet,” Avast wrote.

The stolen data goes to a server in Germany. The same IP address of that server hosts three other domains registered through Naunet.ru, which Avast wrote “is associated with malware and spam activities.”

FileZilla recommended its application be downloaded only from its website or SourceForge, one of its distribution partners. It also recommended to check the SHA-512 hashes of the unmodified version of FileZilla’s installer and executable, which it has published on its blog.

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